1. Field of the Invention
The present invention consists of a visual testing method for observing chromatic aberration between two optical devices. This testing method provides the tester of the optical devices with a noticeable and easily perceivable result that clearly distinguishes an optical device with lower chromatic aberration from that of an optical device with higher chromatic aberration. Furthermore, the test has been designed to allow the tester to distinguish the chromatic aberration properties of two optical devices that have only fine differences in chromatic aberration, such as the differences between ED (extra-low dispersion) and non-ED glass, which may not be visually noticeable in practice except for those highly trained and experienced in the optics field.
2. Description of Related Art
Many optical devices that we use frequently involve the viewing of color: binoculars can be used for enjoyment when users look out over beautiful vistas, for education when bird watchers observe details of a bird's nesting habits, or for utility when hunters track their prey. In all these situations, many colors are input into the optical system, and the optical system has slightly different properties for each color, corresponding to a specific wavelength of light. The term for the variation of the properties of a lens when analyzing over different wavelengths or colors is known as chromatic aberration. Chromatic aberration results in a distortion of the image produced when viewing multi-colored objects through an optical lens, because the lens is unable to focus all colors to the same point. This difference in focal lengths is due to the variation of the index of refraction according to wavelength. As a result of the optical properties varying according to wavelength, red objects focus at different distances than blue objects.
There are numerous examples of complex optical systems or methods that are all aimed at the goal of reducing chromatic aberration, especially computerized systems or specific instrumentation that can measure the chromatic aberration in a system. The data measured is then either used to make corrections in the digital information or to specify how well a new design minimizes chromatic aberration. These methods involve complex and expensive instrumentation to determine chromatic aberration properties, especially aberrations that are not visually observable.
The computerized methods often use either a black circle or another dark shape on a white background. Chromatic aberration is shown in this case as a halo of color around the dark object, with different colors showing at different places around the circle corresponding to the different focal lengths of each wavelength. The computer can then extract the data through computation or the data can be received from the camera in an RBG or equivalent format, where the result is several circles of various colors each at a different offset location than the original black circle. Another method is to highlight a shape or pattern with different colored lights, one at a time, and then compare the recorded information. In regards to visual testing for chromatic aberration, current methods may include looking at objects around the tester, for example, a dark colored boat in the distance on a bright day. The problem is that all these types of testing methods become highly subjective when used as a visual testing method, so that the testing method becomes very inaccurate and useless in systems with low chromatic aberration or small differences in chromatic aberration. For example, in the case of a fairly well chromatically corrected optical system, a test such as the black circle will only produce an extremely thin color halo. The tester then would have to observe the difference in thickness of the halo compared to another device with another very thin border of color, which introduces too much human error. Clearly there is a need for the invention of a new testing method to provide a clear visual result to the tester without the need for computers or instrumentation, which can provide a more quantifiable result.
In regards to visual testing methods in general, there are examples of visual testing methods to measure other optical performance properties such as resolution or contrast, but no current visual testing method exists for chromatic aberration, which doesn't utilizing other equipment or instrumentation. Also, there has been use of visual charts that involve chromatic aberration but are based upon the exploitation of the existence of chromatic aberration, not the measuring or quantifying of the aberration. For example, one application is for use for optometrists or other health professionals, wherein colored charts or letters are presented to the individual who is being tested, as a means of gauging whether their eyeglasses or other lens correction has been adjusted accordingly. When the individual being testing sees a certain color focused more than another, it indicates to the optometrist that their prescription is either over-corrected or under-corrected. The chromatic aberration of the eyeglass is not being tested, just merely used as an indication if there is a need to adjust the focal length of the corrective device.